tag and category crazyness. smart imports?

I know WordPress is smart enough to not duplicate posts when you run an import from another blogging platform (in our case Typepad) and will add comments that have been added since the last import.

Does anybody know more about how this process works?

We have to make a lot of changes to category structure and our tag choices. For example, when we do an import, will the category changes we’ve made since our last import be overwritten?

We’re dealing with a lot tag and category crazyness – it seems like an import plug that let you control what you’re importing would be highly useful.

I’ll go take a look and see what’s there. Just thinking out loud. But any pointers would be great